2023-05-20 15:55:42
Scales, spines, feathers and hairs are examples of skin appendages of vertebrates, which constitute a remarkably diverse group of micro-organs. Despite their natural multitude of shapes, these appendages share early developmental processes in the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently transform the scales that normally cover the legs of chickens into feathers, by specifically modifying the expression of certain genes. These results, published in the journal Scientists progressopen new perspectives for the study of the mechanisms that have allowed radical evolutionary transitions of shape between species.
The skin of terrestrial vertebrates is adorned with various keratinized appendages, such as hair, feathers, and scales. Despite the diversity of shapes within and between species, the embryonic development of dermal appendages generally begins very similarly. Indeed, all these structures develop from cells that produce a localized thickening on the surface of the skin and express particular genes. One of these genes, called Sonic hedgehog (chute), controls a signaling channel – a communication system that allows the transmission of messages within and between cells. Shh signaling is involved in the development of various structures, including the neural tube, limb buds, and skin appendages.
A common ancestor
The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution of the UNIGE Faculty of Sciences, is interested in the physical and biological processes that generate the diversity of appendages in vertebrates. In particular, his group has previously demonstrated that hair, feathers and scales are homologous structures inherited from a common reptilian ancestor.
Chicken embryo feathers are used by scientists as a model system to understand the development of skin appendages. Although some breeds of chickens, such as the ‘Brahma’ and ‘Sablepoot’ varieties, are known to exhibit feathered legs and dorsal foot surfaces, the genetic determinism of this trait is not fully understood.
A temporary change for a permanent change
Since the signaling pathways responsible for this transformation have not been fully determined, Michel Milinkovitch’s group has studied the potential role of the Shh pathway. “We used the classic ‘egg candling’ technique, in which a powerful torch illuminates the blood vessels inside the egg shell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream. ”, explains Rory Cooper, post-doctoral researcher in the laboratory of Michel Milinkovitch and co-author of the study.
The two scientists observed that this specific one-step treatment is enough to trigger the formation of abundant juvenile down-like feathers, in areas that would normally be covered in scales. Remarkably, these experimentally induced feathers are comparable to those covering the rest of the body, as they are regenerative and are then autonomously replaced by adult feathers.
After comparison with embryos injected with a “control” solution (without the active molecule), analysis by RNA sequencing showed that the Shh pathway is activated immediately and persistently following injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales to feathers.
“Our results indicate that an evolutionary leap – from scales to feathers – does not require large changes in genome composition or expression. Instead, a transient change in the expression of a gene, chute, can produce a cascade of developmental events leading to the formation of feathers instead of scales,” explains Michel Milinkovitch. This research, initially focused on the study of the development of scales and feathers, therefore has important implications for understanding the evolutionary mechanisms at the origin of the enormous diversity of animal forms observed in nature.
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